The advent of integrated cone-beam CT on linear accelerators is leading to more detailed information of anatomical motion in treatment position. For instance, large-scale 4D cone beam CT scanning led to the discovery of significant baseline shifts in lung tumor position. This in turn highlighted the necessity of image guidance for lung radiotherapy, in particular for hypo-fractionated schemes.
Our previous application WO2004/066211 described a route to identifying periodic patterns in the two dimensional images of a patient's abdomen from which a cone beam tomography is computed. The two-dimensional images are collapsed along an axis transverse to the cranio-cordal axis to produce a one-dimensional image. Multiple such one-dimensional images are assembled side-by-side in time order to produce a two-dimensional image in which one axis represents time and the other is aligned with the cranio-cordal axis. Periodicities in this image, or a selected region thereof, indicate a periodic movement of a feature in the abdomen in the cranio-cordal direction; the breathing cycle being a principal example. Analysis of the final image can therefore reveal the position in the breathing cycle of a specific image; the images can then be allocated to an appropriate bin and a reconstruction obtained from the images in a specific bin to give an accurate three dimensional image of a selected point in the breathing cycle free from respiration artefacts.